High-frequency wave propagation in the uppermost mantle

Short-period, three-component recordings of the seismic wave field of peace ful nuclear explosions recorded on deep seismic sounding profiles (Quartz a nd Ruby-I, collected in 1984 and 1988, respectively) in northern Russia are used to constrain the nature of the high-frequency teleseismic P-n phase, which can be observed for receiver distances of >3000 km. We suggest that t his phase is caused by velocity fluctuations in the upper mantle acting as scatterers. To test this hypothesis and to determine the properties of the upper mantle scatterers, the elastic reflectivity method (one-dimensional i sotropic models) was used to model the coda of the high-frequency teleseism ic P-n phase. Both the observed data and the synthetic record sections were analyzed by examining the coda decay rates of the teleseismic P-n phase at 5 Hz. Synthetic seismograms are computed for different models based on the global model IASP91 with an added zone of randomly distributed velocity fl uctuations (lamellae) just below the crust-mantle boundary. These models ma y be characterized by the thickness of the scattering layer L, the vertical heterogeneity correlation length a, and the heterogeneity standard deviati on sigma. The numerical simulation of the wave propagation in these models generated a high-frequency teleseismic P-n phase with a well-pronounced, lo ng coda. A comparison of the modeling results with the observations shows c learly that a velocity model containing fluctuations in the upper mantle ca n easily explain the presence of a high-frequency teleseismic P-n. Comparin g the coda decay rates of synthetic seismograms with the observation, we tr ied to constrain the properties of the velocity fluctuations in the upper m antle, L, a, and sigma. In the distance range of the uninterfered high-freq uency teleseismic P-n phase beyond 1300 km, the coda decay rates for our be st fitting model are similar to the observed ones. This model has a 75 km t hick zone of scatterers below the Moho, containing lamellae with an average thickness of 2 km and a RMS velocity perturbation of 5%. The modeling I re sults show also that two other possible models, the whispering gallery phas e along the crust-mantle boundary and scattering in the lower crust, taken alone are not able to explain the coda properties of the high-frequency tel eseismic P-n phase.